Emblem of Conquest vendors can be found all over Dalaran in the various weapon and armor shops selling the appropriate armor for their location, as well as in the Alliance and Horde quarters of Dalaran:

/Patch 4.0.1 (12-Oct-2010): Removed.Emblems of Conquest will be immediately converted to gold at a ratio of 5 50 per emblem. Items which were purchased using Emblems of Conquest now require Justice Points instead.